Heating, ventilating, and air-conditioning (HVAC) systems have been used to maintain desirable temperatures and humidity levels within buildings, and buildings have been constructed with ventilation systems including HVAC systems to provide comfortable and safe environments for occupants to live and work. To maintain fresh air within buildings and to reduce the level of indoor air contaminants, in many applications, at least a portion of the air handled by ventilation or HVAC systems has been taken from outdoors, while a portion of the indoor air handled by HVAC systems has been exhausted to outside the building. In many applications, particular sources of contaminants within buildings are located near exhaust vents or hoods so that a high percentage of the contaminants produced are exhausted by the ventilation systems out of the building.
However, in most situations, outside air introduced to the building to replace exhaust air must be cooled or heated before being introduced to the building to provide temperatures within desired parameters, and often must be dehumidified or humidified in order to keep humidity levels within desired ranges. But adding or removing heat or humidity (moisture) typically involves the expenditure of energy. To reduce the energy required to condition the outside air, recovery wheels have been used to transfer heat, moisture, or both, between exhaust air and incoming outside air. Examples of the prior art in these areas are described in U.S. Pat. Nos. 4,769,053 and 6,199,388, and U.S. Patent Application publication number 2004/0000152, all by the same inventor as this document, and all of which are incorporated herein by reference in their entirety. These prior art documents also describe many of the needs and benefits of such systems and the use of recovery wheels.
When a recovery wheel is used, air may be exchanged from one side of the wheel to the other. In applications where exhaust air may be highly contaminated, such as exhaust from a laboratory hood or a source of contaminants, it may be desirable that the exhaust air, and contaminants therein, not be permitted to transfer in large or significant quantities into the outside air or supply air that is to be introduced into the building. To accomplish this, the pressure of the outside air may he higher than the pressure of the exhaust air at the recovery wheel, and a portion of the outside air may be diverted to purge the return or exhaust air from a portion of the rotating recovery wheel before that portion passes from the return/exhaust side of the wheel to the outside/supply air side of the wheel. A portion of the outside air may be diverted to form purge air using a baffle plate, for example, which may be made of sheet metal, for instance. The baffle plate or purge section of the recovery wheel typically uses the pressure differential between the outside/supply airstream and the return/exhaust airstream, to force high velocity, unconditioned, clean outside air through the wheel media as it rotates from the dirty return/exhaust airstream into the clean outside/supply airstream.
In this way, some or all of the exhaust air contaminants that would otherwise be trapped within the volume of the wheel transfer media as it rotates from the contaminated airstream into the clean airstream are forced or directed back into the return air steam and subsequently exhausted outdoors. As a result, the air that is contained within the transfer media, as it passes into the outside/supply airstream, may be substantially uncontaminated, clean air.
FIGS. 1a and 1b illustrate an example of a purge section as it has been employed in the prior art. In the side view of FIG. 1a, recovery wheel 10 rotates about axle 11 in the direction shown, transferring heat, moisture, or both, between return/exhaust airstream 16 and outside/supply airstream 14. Outside/supply airstream 14 starts in the supply system as outside air 14a, typically from outside the building, some of which is diverted through purge section 12 into return/exhaust airstream 16, forming purge flow 14c. Except for purge airflow 14c, outside air 14a becomes supply air 14b after being at-least partially conditioned by passing through recovery wheel 10. Return/exhaust airstream 16 in the exhaust system is return air 16a upstream of recovery wheel 10 and becomes exhaust air 16b after passing through recovery wheel 10 and receiving purge airflow 14c. Return/exhaust airstream 16 includes contaminants 18, in this example, which may be harmful or unpleasant contaminants, for instance. Absent purge section 12, the rotation of wheel 10 would tend to carry some contaminants 18 into outside/supply airstream 14, but with purge section 12, purge airflow 14c removes these contaminants 18 from wheel 10 and returns them (along with purge airflow 14c) to return/exhaust airstream 16. Outside/supply airstream 14 and return/exhaust airstream 16 (and the supply and exhaust systems) are separated by duct wall 15 in this illustration.
FIG. 1b is an end view of the recovery wheel 10 of FIG. 1a, illustrating purge angle 19 of purge section 12. Provided that the system pressurization is set appropriately, the purge angle 19 is correctly selected, and the appropriate purge pressure is maintained, any cross-contamination associated with air trapped within the transfer media of wheel 10 has been limited to or below 0.045% of the return air 16a concentration of contaminant 18 for certain recovery wheels 10. This carry-over percentage is well below a 0.1% level of short circuiting frequently observed as a result of normal re-entrainment between the exhaust outlet and outside air intake of well-designed facilities such as laboratories.
Even with recovery devices such as recovery wheels (e.g., 10), HVAC systems still use a considerable amount of energy, including both for moving air (fan power) and further conditioning of supply air 14b. To further reduce energy consumption, HVAC systems have been developed that reduce airflow when less flow is needed (variable air-volume systems). In many applications, variable-frequency drive systems have been provided for fan motors, with variable-speed controllers, to reduce fan speeds when full air volume is not needed. Variable voltage DC drives are another option. In some cases, a number of fans are used, and a portion of the fans are shut off when less flow is needed rather than (or in addition to) varying the speed of the fans. Variable air-volume systems have been used that reduce fan power required, that reduce the volume of outside air that must be conditioned, or both. But problems have been encountered when variable air-volume systems have been used in conjunction with recovery systems and devices such as recovery wheel 10.
Specifically, applying energy recovery wheels and maintaining acceptable contaminant carry-over levels in variable air-volume systems has shown to be rather difficult due to changes in system pressures, airflows, or both. Without a way to ensure effective purge operation, cross-contamination has resulted from 3% to as high as 20% of the exhaust air concentration for variable airflow systems incorporating energy recovery wheels. High carry-over percentages have resulted when the quantity of contaminated air transferred increases due to a reduction in the purge airflow (and thereby purge efficiency), for example, at the same time that the supply airflow is being reduced.
For applications such as research laboratories, hospitals, smoking areas and those involving toilet exhaust, just to name a few, a problem has existed that this level of cross-contamination is unacceptable. Therefore, a need or potential for benefit has existed for effective purge operation in variable air-volume systems. In fact, needs or potential for improvement have existed so that an industry standard for these types of applications can be reached that may be as stringent as 0.1%, with laboratory applications often desiring 0.01%. Thus, and for reasons that will become apparent, systems and methods to better ensure effective purge operation for energy recovery wheel systems operated with variable airflow volume (without requiring excessive purge airflow at the peak airflow design conditions) is a top priority for many end users and mechanical consultants.
In a partial solution to this problem, purge angle 20 has been set for worst-case conditions, typically for minimum flow, minimum pressure differential between outside/supply airstream 14 and return/exhaust airstream 16, or both. However, this results in excessive (more than needed) purge airflow 14c under other conditions, for example, at higher flow. Excessive purge airflow 14c not only increases fan power that is required, but also reduces the effectiveness of recovery wheel 10 at transferring heat, moisture, or both, between return/exhaust airstream 16 and outside/supply air 14. Thus, needs or potential for benefit exist to provide for appropriate purge flow (sufficient but not excessive). Needs or potential for benefit exist in a ventilation system, to maintain purge effectiveness, limit carry-over of contaminants, and yet limit (e.g., to what is necessary) or minimize purge airflow. Needs or potential for benefit exist to control recovery systems such as recovery wheels, control ventilation systems, modify existing ventilation systems, provide control systems for recovery wheels and ventilation systems, provide ventilation systems, and provide buildings with ventilation systems that maintain purge effectiveness, limit carry-over of contaminants, and yet limit or minimize purge airflow.
Due to the increasing cost of electricity, the desire to reach energy efficiency, needs or desire for compliance with energy codes and standards (e.g., ASHRAE 90.1) many facilities would benefit from the ability to incorporate variable air-volume distribution systems. If this problem of purge inefficiency is not effectively addressed, many applications that could benefit significantly from the use of recovery wheels will not be able to do so since even 3% carry-over is often considered to be unacceptable in certain applications. Potential for improvement exists in these and other areas that may be apparent to a person of skill in the art having studied this document.